1. Field of the Invention
The present invention relates to a method for producing a solid-state imaging device.
2. Description of the Background Art
In a solid-state imaging device, it is very important to suppress the occurrence of a dark current near photodiodes, in terms of improving the performance of the solid-state imaging device. Therefore, various methods or devices for suppressing the occurrence of such a dark current have been proposed. Hereinafter, an example of a conventional solid-state imaging device which suppresses the occurrence of a dark current will be described with reference to the figures. FIG. 6 is a view showing a cross-sectional structure of the solid-state imaging device. The right-hand side of FIG. 6 shows a light-receiving region in which photosensitive elements and the like are formed. The left-hand side of FIG. 6 shows a peripheral wiring region in which wiring and the like for the solid-state imaging device are formed.
The solid-state imaging device shown in FIG. 6 comprises a silicon substrate 1, a gate insulating film 2, a transfer electrode 3, an interlayer insulating film 4, a light-shielding film 5, an insulating film 6, a wiring film 7, an insulating film 10, a microlens 11, and a photodiode (hereinafter abbreviated as “PDs”) 12. The solid-state imaging device is produced through steps described below.
First, a PD 12, which is a photosensitive element, is formed on the silicon substrate 1. Next, on the silicon substrate 1, a CCD circuit light-receiving section is formed which is composed of: the gate insulating film 2 (e.g., a silicon oxide film), the transfer electrode 3 (e.g., polysilicon), the interlayer insulating film 4 (e.g., a silicon oxide film), the light-shielding film 5 (e.g., W(tungsten)), and the like. Thereafter, the insulating film 6 (e.g., a silicon oxide film) for providing insulation between a lower layer wiring (not shown) and the wiring film 7 is deposited. On the insulating film 6, the wiring film 7 (e.g., Al (aluminum)) is formed by a photolithography technique using a resist film.
Once the formation of the wiring film 7 is completed, the insulating film 10 (e.g., silicon nitride film), which contains plenty of N(nitrogen)-H(hydrogen) bonds, is formed on the insulating film 6 and the wiring film 7 by a plasma CVD (Chemical Vapor Deposition) technique. Thereafter, a heating process is performed for the solid-state imaging device at 300° C. to 450° C. for several minutes or several hours. Due to the formation of the insulating film 10 and the heating process, the dark current is reduced. Hereinafter, the reason why the heating process reduces the dark current will be described in detail.
A dark current occurs due to, for example, an interface state occurring at an interface between the silicon substrate 1 and the gate insulating film 2, which is a silicon oxide film. The interface state causing the dark current is ascribable to the presence of unaccomplished bonds (hereinafter referred to as “dangling bonds”) of silicon atoms existing at an interface between the silicon substrate 1 and the gate insulating film 2, which is a silicon oxide film. In other words, it is possible to suppress the occurrence of a dark current by reducing the number of dangling bonds.
Therefore, in the conventional solid-state imaging device above, hydrogen atoms are bound to dangling bonds of silicon atoms in order to reduce dangling bonds, thus reducing the interface state. Specifically, after depositing the insulating film 10, which is a silicon nitride film containing N—H bonds, a heating process is performed in order to allow the hydrogen atoms contained in the insulating film 10 to diffuse into the solid-state imaging device. The hydrogen atoms diffusing into the solid-state imaging device will pass through the insulating film 6 and the like, and arrive at the interface between the silicon substrate 1 and the gate insulating film 2. Then, the hydrogen atoms bind to the dangling bonds present at the interface between the silicon substrate 1 and the gate insulating film 2. As a result, the interface state occurring at the interface between the silicon substrate 1 and the gate insulating film 2 can be reduced, whereby the occurrence of a dark current can be suppressed (see, for example, FIG. 6 of Japanese Laid-Open Patent Publication No. 5-283667).
The silicon nitride film which has been formed through a plasma CVD technique used for the insulating film 10 has a higher optical absorption rate than does any other insulating film. Therefore, if the insulating film 10 is present below the microlens 11, the amount of light reaching the PD 12 will be reduced, thus deteriorating the sensitivity characteristics of the solid-state imaging device. Therefore, from the standpoint of improving the sensitivity of the solid-state imaging device, it is preferable to remove the insulating film 10 through an etching process, after performing the aforementioned heating process.
In order to remove the insulating film 10 through an etching process, it is necessary to perform the etching process after providing a mask on the wiring layer 7. The reason is that the wiring layer 7 would be damaged if the wiring layer 7 were directly exposed to an etching process.
However, forming a mask in the region of the wiring layer 7 would require a large number of complicated steps, such as application of a photoresist, an exposure, and a development. Thus, the fact that a mask for protecting the wiring layer 7 is needed during a step for removing the insulating film 10 implies an increase in the number of steps for producing the solid-state imaging device.